1. Field of the Invention
This invention relates generally to the manufacture of semiconductor wafers prepared by a method including applying a photo-resist layer, exposing the layer, and stripping the layer from the semiconductor wafer. More particularly, this invention pertains to a method for inspecting semiconductor wafers or other substrates to determine the presence of residual photo-resist material on the semiconductor wafer surface.
2. State of the Art
Semiconductor chips are produced in a multi-step process by which a plurality of identical electronic circuits is typically formed on a semiconductor substrate, such as a silicon wafer. The semiconductor substrate is then subdivided (diced) into individual chips which are further processed into semiconductor devices.
The electronic circuits are generally patterned into a semiconductor wafer by lithography. In this process, a resist material is coated onto the semiconductor wafer surface. As disclosed in commonly owned U.S. Pat. No. 5,350,236, issued Sep. 27, 1994, hereby incorporated herein by reference, the application of a material on a semiconductor substrate can be monitored by measuring light reflected from a surface of the semiconductor substrate.
After the resist material has been coated on the semiconductor wafer surface, it is selectively exposed to a radiation source, such as by the passage of radiation (i.e., light, e-beam, or X-rays) through a mask having the desired pattern. Some portions of the resist receive a high dosage of radiation while other portions receive little or no radiation, resulting in a difference in solubility from the resist portions. In a subsequent development step, a developer removes or etches portions of the resist coating from the semiconductor substrate at a rate higher than other portions. The selective removal results in a resist pattern which will become the electronic circuit pattern on the semiconductor substrate. Precision in the development time is critical for achieving complete removal of resist from some portions while leaving other portions substantially intact. Both insufficient development and excessive development will result in a lack of differentiation, forming a defective electronic circuit pattern on the semiconductor substrate. In addition, where the width of a conductor line(s) in the electronic circuit is critical, inadequate development results in an overly narrow line, and excessive development produces an overly wide line. Thus, precise endpoint detection (i.e., the moment at which precise development occurs) is a requirement for proper development.
Following the removal of the portions of the photo-resist material in the development process, the semiconductor wafer is subjected to further processing steps which may include doping, etching, and/or deposition of conductive materials in unprotected areas, i.e., areas devoid of photo-resist material. After one or more of these processing steps, the semiconductor wafer is subjected to a stripping step to remove the photo-resist material remaining on the semiconductor wafer.
After the removal of the photo-resist material, a subsequent processing step may include heating the semiconductor wafer in a diffusion furnace or applying a layer of material with a chemical vapor deposition system. Occasionally, a semiconductor wafer is inadvertently passed to a thermal furnace or vapor deposition system without removal or with only partial removal of the photo-resist material. The resulting damage to the processing equipment may be severe. For example, furnace diffusion tubes are irreparably damaged by vaporized hydrocarbons and carbon from the photo-resist material and, thus, the furnace diffusion tubes must be replaced. The replacement equipment and/or the downtime to repair the processing equipment is usually very costly.
Furthermore, the photo-resist carrying semiconductor wafer and one or more subsequent semiconductor wafers entering the processing equipment prior to shutdown of the equipment are usually also contaminated and must be discarded. At a late stage of manufacture, a semiconductor wafer may have a value between about $10,000 and $20,000. Thus, even an occasional loss is significant.
One method used in the industry to detect such residual photo-resist material is manual inspection with a microscope. However, manual inspection of semiconductor wafers to detect photo-resist materials has not been sufficiently effective. First, photo-resist is typically difficult to see using a conventional white light microscope, and even an experienced microscopist may inadvertently miss photo-resist on a wafer. Secondly, since manual inspection is laborious and time-consuming, it is generally not cost-effective to manually inspect more than a very small number of the semiconductor wafers (usually less than 10%). Thus, unstripped semiconductor wafers may still be missed by manual inspection.
Accordingly, an object of the present invention is to provide an improved method for rapid automated detection of resist material on semiconductor wafers in order to reduce process downtime, material wastage, maintenance/repair expenses and production costs.
The present invention is an automated method and apparatus for determining the presence or absence of a photo-resist material on the surface of a semiconductor substrate by the detection of fluorescence, reflection, or absorption of light by the photo-resist material.
Photo-resist materials are generally organic polymers, such as phenol-formaldehyde, polyisoprene, poly-methyl methacrylate, poly-methyl isopropenyl ketone, poly-butene-1-sulfone, poly-trifuluorethyl chloroacrylate, and the like. Organic substances can generally fluoresce (luminescence that is caused by the absorption of radiation at one wavelength followed by nearly immediate re-radiation at a different wavelength) or will absorb or reflect light. Fluorescence of the material at a particular wavelength, or reflection/absorption by the material of light at a given wavelength, may be detected and measured, provided the material differs from the underlying semiconductor substrate in fluorescence or reflection/absorption at a selected wavelength or wavelengths. For example, a positive photo-resist generally fluoresces red or red-orange and a negative photo-resist generally fluoresces yellow.
In a particular application of the invention, the presence of photo-resist material on a semiconductor wafer surface may be rapidly and automatically determined, recorded, and used to drive an apparatus which separates semiconductor wafers based on the presence or absence (or quantity) of the photo-resist material. Thus, semiconductor wafers which have been incompletely stripped of photo-resist material (or not stripped at all) may be automatically detected and culled from a manufacture line of fully stripped semiconductor wafers and reworked. Thus, contamination of downstream processes by unstripped semiconductor wafers is avoided.
In this invention, the semiconductor wafer is irradiated with light which may be monochromatic, multichromatic, or white. In one version, the intensity of generated fluorescence peculiar to the photo-resist material at a given wavelength is measured. In another version, the intensity is measured at a wavelength which is largely or essentially fully absorbed by the photo-resist material. In a further variation, the intensity of reflected light is measured at a particular wavelength highly reflected by the photo-resist material but absorbed by the substrate.
The intensity of fluoresced or reflected light is measured by a sensing apparatus and the result is put to a logic circuit, e.g., a computer. The result may be recorded and used for a decision making step and control of a robotic device. The robot performs the semiconductor wafer handling tasks, such as transferring the semiconductor wafers from a semiconductor wafer cassette to an inspection stage, and transferring the inspected semiconductor wafers to a destination dependent upon the test results.
A permanent record of the test results may be automatically retained and printed, and semiconductor wafers identified as being partially or totally unstripped or otherwise abnormal or defective are separated for proper disposition.
The apparatus for conducting the detection test process is generally comprised of known components which in combination produce accurate results in a very short time without laborious manual inspection. A high test rate may be achieved in a continuous or semi-continuous manufacturing process, enabling all product units to be tested. The current laborious and time-consuming testing of a few random samples by manual microscopic inspection methods is eliminated. The test results are in electronic digital form and may be incorporated into a comprehensive automated manufacturing documentation/control system.
The test apparatus may comprise a stand-alone system through which individual substrate units are passed for a separate detection/measurement step. Thus, for example, following a stripping step, semiconductor wafers may be moved sequentially through the test apparatus for confirmation of full stripping, and for culling of non-stripped semiconductor wafers.
In another version of the invention, the test apparatus may be incorporated into a processing step such as embodied in a resist stripping device for in situ determination of residual resist material on semiconductor wafers undergoing stripping. The stripping end-point may be thus determined and may be used to activate automated transfer of the stripped wafers from the resist stripper to the following process step when stripping is complete. This embodiment is particularly adaptable to plasma and wet-stripping apparatuses.
While the method and apparatus are particularly described herein as relating to the detection of photo-resist material in a lithographic process, they may also be used to detect the presence and quantity of any material on a semiconductor substrate, where the material and semiconductor substrate have differing fluorescing/absorbing properties at a given selected wavelength of radiation. The material may be an organic substance having naturally fluorescing properties under a particular spectrum of radiation, or may be a substance with little natural fluorescence, spiked with a material which fluoresces when irradiated with light of a particular wavelength.